The numbers didn’t add up to David Baumann. How could the proposed 200-megawatt Sugarland Wind project in Florida be expected to produce as much energy as the 200-megawatt Top of the World wind farm in Wyoming?

You didn’t have to be a wind engineer to see that Converse County, outside Casper, Wyo., is a heck of a lot windier than anywhere in Florida. And yet the Sugarland developers were claiming that their Palm Beach County wind farm would meet the power needs of “up to 60,000 U.S. households” – same as what Duke Energy said about Top of the World when it began operating in 2010.

image via NREL

So Baumann penned a piece for the Berkshire (Mass.) Eagle with the headline, “Wind power numbers just don’t add up,” and in the piece he said wind companies “flippantly throw out boilerplate numbers that have no basis in scientific fact.”

He might be right – in some cases.

Baumann’s article led us to poke at the “number of household” claims that utilities and developers make for big projects, both solar and wind. What we found is that sometimes they’re solid, and sometimes they’re not.

First, though, we should explain what is and isn’t meant by a households figure. It’s not how many households a project powers at any given moment in time – wind power production waxes and wanes, as does household energy use, so a single number would be difficult if not impossible to calculate. Instead, the households figure tells you how many homes could be fully powered by a project in a year based on a project’s annual production.

So let’s start with Sugarland. It’s impossible to know exactly how the developers, a company called Wind Capital Group, came up with the 60,000 figure; they never returned the messages we left asking for more information.

But here’s what we can say: Based on the Energy Information Administration’s figure of 11,496 kilowatt-hours of energy use per U.S. household per year, 60,000 homes use 689,760,000 kWh in a year, which can also be expressed as 689,760 megawatt-hours/year. A 200 MW wind farm, going at full tilt 24 hours a day, 365 days a year, would produce 1,752,000 MWh annually – but, of course, that doesn’t happen. Wind farms sometimes work near 100 percent capacity, other times at 10 percent, other times at 60 percent — it varies throughout the day. The percentage of the total possible production that a project actually yields in a period of time is called the capacity factor. According to a recent federal study, wind power plants that went into operation in 2010 had an average capacity factor of 33.5 percent.

If the Florida plant were an average performer, then, it would produce 586,920 MWh in a year – enough to power 51,000 average U.S. homes. But it is highly unlikely a wind farm in Florida will produce anywhere near that much power. A 2008 study prepared for the state of Florida [PDF] noted that “no Class 3 regimes, which are generally the minimum for economically viable wind farms, have been identified,” in Florida.

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